Usually an optical system such as a digital camera and a mobile phone with camera is structured with a combination of the plurality of lenses. In cases of assembling these optical systems, by precisely matching optical axes of the plurality of lenses with each other, structuring the lens assembly by fixing between the lenses in matched state, after that, the image capturing device is structured by arranging an image capturing sensor on the lens assembly. In this case, the optical system with displaced optical axes causes the problems of image distortion, blurred image and the like, which cannot cause a good imaging performance. For example, with respect to a camera lens unit including the plurality of lenses it is required to assemble the plurality of lenses in high precision, as image sensors being developed to be higher density of pixels, and the assembling process is becoming difficult. For example, in the work of assembling the plurality of lenses, an axis aligning work to match the axis of each of the lenses is included, and to precisely match each axis of the lenses by the aligning work is a work of very high difficulty level. As the method of assembling the plurality of lenses, there are mainly four types of assembling methods as shown below. Said four types of assembling methods will be described referring to FIGS. 14a-14d, which are section views of the lens showing the assembling methods relating to the conventional methods.
A first method is a lens frame fitting assembling method. This is a method of, as shown in FIG. 14a, tucking the side surfaces of first lens 500 and second lens 51 in lens frame 520, and assembling by physically positioning the two lenses at the reference position of lens frame 520. Since the positioning is executed by lens frame 520, the two lenses can be easily assembled without executing the aligning work to match the light axes of first lens 500 and second lens 510. However, since the positioning of two lenses is conducted by the use of separate other part such as lens frame 520, the problem of inferior positioning precision is caused.
A second method is a reference position fitting assembling method. In this method, as shown in FIG. 14b, flange 531 is provided in outer circumferential area outside of an effective optical surface of the first lens 530. Similarly, flange 541 is provided in outer circumferential area outside of an effective optical surface of the second lens 540. Then, by fitting the first lens 530 and the second lens 540, flanges 531 and 541 function as reference positions and the physical positioning of the two lenses is performed to enable the assembling of the two lenses. According to the second method, since the positioning of the two lenses is physically executed with flanges (reference position), high precision assembling can be easily performed without executing the aligning work to align the light axes. Since the flanges function as the reference positions, the positions of the flanges need to be precisely provided on each lens, however high precision formation of the flanges is very difficult. Namely, producing the lens having high precision reference position is a work of high difficulty level. Further, conducting the assembling work with measuring the reference position (flange position) is also very difficult.
As a third method, there is a method of assembling lenses while aligning the light axes by measuring the optical performance of the lenses. For example, as shown in FIG. 14c, by providing the first lens 550 on the second lens 560 to form a lens assemble, the aligning is performed by moving the first lens 550 in parallel direction (arrow direction) while measuring the optical performance of the lens assembly. Then, by determining the position where optical performance of the lens assembly becomes good, the first lens 550 and the second lens 560 are fixed. According to this assembling method, although the assembling can be performed with high precision, a problem is caused that long time is required for the axes aligning work including the measuring of the optical performance. Further, this assembling method is only applicable to the case of assembling the lens assembly that can be measured of optical performance, which causes a problem of lacking versatility. For example, in the case of assembling a lens unit having plural lenses, since this method can be utilized only for the case of assembling the last lens, this method lacks versatility.
As a forth method, there is a method of assembling lenses while aligning the light axes by utilizing the mark formed on the lens. For example, as shown in FIG. 14d, by providing mark 571 on the first lens 570 at the surface facing to the second lens 580, and providing mark 581 on the second lens 580 at the surface facing to the first lens 570, reading the mark position of each lens, and matching the position of mark 571 with the position of mark 581 by moving the first lens 570 in parallel direction (arrow direction), the axis aligning of the first lens and the second lens is executed. By utilizing the marks in this way, the axis aligning can be easily executed, however since this alignment is only for the parallel direction, to precisely align the light axis is difficult.
Namely, since the alignment is only for the parallel direction, tilt is generated between the first lens 570 and the second lens 571. According to the conventional method, the alignment is performed only in the parallel direction, and the tilt cannot be adjusted. With respect to the tilt, explanation will be done by referring to FIG. 15. FIG. 15 is a section view of a lens assembly relating to the conventional technology. According to the above described forth method, since the adjustment is done only for the parallel direction of the lens, the tilt of axis having tilt angle φ is generated between the first lens 570 and the second lens 580, and this angle φ cannot be adjusted. According to the method of conventional technology, for example, the tilted axis of 5-10 arc minutes (5′-10′) is generated, which cannot satisfy the tilt angle required for an optical system of 12 million pixels class, for example.
Further, a method of assembling lenses by forming a mark at an outer peripheral area outside the effective optical surface of lens and utilizing the mark is known (for example, Patent Document 1). A method of assembling lenses by utilizing a deficiency existing at a prescribed position in the effective optical surface of the lens as a mark for positioning is known (for example, Patent Document 2). Further, a method of executing the lens alignment by forming a mark on the lens is known (for example, Patent Document 3).